Recently, there has been a strong demand for reduction of the amount of carbon dioxide in order to deal with global warming. In the automobile industry, the reduction of emissions of carbon dioxide is highly expected in association with the spread of electric vehicles (EV) and hybrid electric vehicles (HEV). Thus, development of electrical devices such as secondary batteries for driving motors as a key to practical application of such vehicles is actively being carried out.
The secondary batteries for driving motors are required to have quite high output performance and high energy as compared with lithium ion secondary batteries for general use in mobile phones, laptop computers and the like. Therefore, lithium ion secondary batteries having the highest theoretical energy among all types of batteries are gaining more attention, and they are now being rapidly developed.
A lithium ion secondary battery generally has a constitution in which a positive electrode including a positive electrode current collector to which a positive electrode active material and the like is applied on both surfaces with use of a binder is connected, via an electrolyte layer, to a negative electrode including a negative electrode current collector to which a negative electrode active material and the like is applied on both surfaces with use of a binder, and the battery is housed in a battery case.
In a lithium ion secondary battery of a related art, a carbon graphite-based material, which is advantageous in terms of charge and discharge cycle life or cost, has been used for the negative electrode. However, the carbon⋅graphite-based negative electrode material has the disadvantage that a theoretical charge and discharge capacity equal to or larger than 372 mAh/g, which is obtained from LiC6 as a compound introduced with maximum amount of lithium, cannot be ensured because the battery is charged and discharged by absorbing lithium ions into graphite crystals and desorbing the lithium ions therefrom. Thus, by use of the carbon⋅graphite-based negative electrode material, it is difficult to ensure a capacity and energy density that are high enough to satisfy vehicle usage on the practical level.
On the other hand, a battery using a SiOx (0<x<2) material, which can form a compound with Li, for a negative electrode has a higher energy density than the carbon⋅graphite-based negative electrode material of a related art. Therefore, such a negative electrode material is highly expected to be used for a battery in a vehicle. For example, in silicon oxide having a chemical composition of SiOx, Si (nanoparticles of monocrystal) and non-crystalline (amorphous) SiO2 are present as separate phases when it is observed at microscopic level.
The silicon oxide has a tetrahedral structure as a unit structure. Silicon compounds other than SiO2 (intermediate oxide) can be expressed as Si2O, SiO, or Si2O3 corresponding to oxygen number of 1, 2, or 3 at the corner of the tetrahedron. However, as these intermediate oxides are thermodynamically unstable, it is very difficult for them to be present as a monocrystal. Thus, SiOx has a non-crystalline structure in which the unit structures are randomly arranged, and such a non-crystalline structure is formed such that plural non-crystalline compounds are present without forming an interface, and it is mainly composed of a homogeneous non-crystalline structure part. Thus, SiOx has a structure in which Si nanoparticles are dispersed in non-crystalline SiO2.
In the case of such SiOx, only Si is involved with charging and discharging, and SiO2 is not involved with charging and discharging. Thus, SiOx indicates average composition of them. In SiOx, while 1 mol of Si absorbs and desorbs 4.4 mol of lithium ions in accordance with the reaction formula (A) and a reversible capacity component of Li22Si5 (=Li4.4Si) with a theoretical capacity of 4200 mAh/g is generated, there is a significant problem that, when 1 mol of a SiO absorbs and desorbs 4.3 mol of lithium ions in accordance with the reaction formula (B), Li4SiO4 as a cause of having irreversible capacity is generated together with Li4.4Si during initial Li absorption.
[Chem. 1]Si+4.4Li++e−Li4.4Si  (A)4SiO+17.2Li→3(Li4.4Si)+Li4SiO43Si+13.2Li+Li4SiO4  (B)
Meanwhile, examples of a lithium silicate compound containing Li include LiySiOx (0<y, 0<x<2) such as Li4SiO4, Li2SiO3, Li2Si2O5, Li2Si3O8, and Li6Si4O11. However, since these LiySiO), have very small electron conductivity and SiO2 has no electron conductivity, there is a problem of having increased negative electrode resistance. As a result, it becomes very difficult for lithium ions to get desorbed from a negative electrode active material or get absorbed into a negative electrode active material.
However, in a lithium ion secondary battery using the material alloyed with Li for the negative electrode, expansion-shrinkage in the negative electrode is large at the time of charging and discharging. For example, volumetric expansion of the graphite material in the case of absorbing Li ions is approximately 1.2 times. However, the Si material has a problem of a decrease in cycle life of the electrode due to a large volumetric change (approximately 4 times) which is caused by transition from an amorphous state to a crystal state when Si is alloyed with Li. In addition, when using the Si negative electrode active material, the battery capacity has a trade-off relationship with cycle durability. Thus, there has been a problem that it is difficult to increase the capacity and improve the cycle durability concurrently.
In order to deal with the problems described above, there is known a negative electrode for a lithium ion secondary battery containing SiOx and a graphite material (for example, see Patent Literature 1). According to the invention described in the Patent Literature 1, it is described in paragraph [0013] that, by having SiOx at minimum content, not only the high capacity but also good cycle lifetime can be exhibited.